The loss of bone strength, cardiovascular function, and muscle atrophy are the primary health-related concerns associated with long-term space flight. The probable cause of bone demineralization and muscle atrophy in space is the reduction in the levels of force required to perform activities, although other factors, unique to gravity- or acceleration-free environments, such as fluid shifts and the loss of fluid hydrostatic pressure gradients, may also exert a systemic influence. Presently, exercise protocols and equipment for space flight are unresolved, although recent calculations suggest that all exercise in space to date has lacked sufficient loads to maintain pre-flight musculoskeletal mass.
Gravity and one's daily physical activity level on Earth combine to impose a unique history of external and internal forces on the body. The time history of the muscle, bone, and cardiovascular tissue stresses determines to a large degree the material, geometric, and physiological properties of musculoskeletal and cardiovascular tissue. Altering the form and intensity of daily activity while in space can be expected to cause long term changes in the morphology and physiology of these tissues, as the evidence from space indicates.
A treadmill, in which the exerciser is connected to the cabin floor by elastic (bungy) cords attached at the waist and shoulders, is the principal exercise device used in space by both Soviet cosmonauts and U.S. astronauts. The treadmill provides cardiovascular exercise and musculoskeletal loading principally to the lower limbs. Walking and running on the treadmill have not been completely effective in maintaining musculoskeletal tissue, probably because the forces developed by the elastic cords are not equivalent in both magnitude and manner of application to the force of gravity on Earth. The application of force by the cords is uncomfortable and causes early fatigue.